The present invention relates to a method of producing a solution of a polymer or copolymer of vinyl chloride series and especially to a method of producing such polymer or copolymer solution in the preparation of having utility film or synthetic fiber.
In the prior art, there are known two methods for preparing a solution of vinyl chloride polymer. In one method, emulsion or suspension polymerization of vinyl chloride or a mixture of vinyl chloride and one or more monomers copolymerizable therewith is carried out in an aqueous medium and the obtained polymer is dissolved in an organic solvent. In the other method, solution polymerization of the foregoing monomers is carried out in an organic solvent which can dissolve the obtained polymer or copolymer.
In the first case, though there are many advantages, such as availability of proven techniques for emulsion and suspension polymerization, high polymerization velocity, high productivity, and ease of producing polymers of high molecular weight, etc, those methods are disadvantageous and labor consuming since it is necessary to employe such steps as salting out, dehydration, washing and drying to obtain a purified product. Moreover, it is necessary to dissolve the dried product in a solvent. Accordingly, the plant cost is high. Moreover, exhausting of the emulsifying agent raises the possibility of contamination of the waste water line.
In the latter method, although such disadvantages as mentioned above may not exist, since a polymer solution already prepared for spinning is obtained at the time when polymerization reaction is completed, unfortunately, the polymerization velocity in solution is very slow and the degree of polymerization of the obtained polymer is too low for various purposes such as those contemplated herein. When a single component polymerization or co-polymerization of vinyl chloride monomer is carried out in N,N-dimethyl formamide (hereinafter called "DMF," for sake of convenience), using usual initiators for free radical polymerization, ordinarily the polymerization velocity is very slow and the degree of polymerization of the obtained polymer is too low. This may be due to chain transferring ability of DMF.
Japanese Patent Publication 36-10996 disclosed a modified process for polymerizing acrylonitrile in which a mixture of water and an organic solvent is employed in place of organic solvent alone. However, this process when applied to polymerization of vinyl chloride or a monomer mixture having vinyl chloride as the predominant constituent meets with significant problems. When applied to a system having high monomer or monomers content, the polymer produced is deposited in a state of powder to form a dispersal phase and further polymerization becomes difficult to continue because the system is filled up with polymers of low bulk density at an early stage of polymerization. On the other hand, when applied to a system having low monomer content, the polymerization proceeds in a solution state and the polymerization proceeds very slowly. The obtained product is one having low degree of polymerization.